It is important in the treatment of patients that they receive the correct dosage of drugs or saline drips at a defined rate. This is done by medical infusion devices of which there are many types ranging from motorized hypodermics and small diaphragm cassettes for low flow rates of 1 milliliters per hour to drip feeds for higher flow rates of say 1 liter per hour. However it is necessary to test the infusion device to ensure its performance. Since especially the low flow rate devices have an intermittent flow rate, the flow can be described as "lumpy". While it would be possible to have a testing device for each type of infusion device, it is desirable to have a common testing device for as many types of infusion devices as possible. U.S. Pat. No. 4,938,072 describes a testing device for a range of infusion devices. The device is the optical measurement type sensing when a liquid level in a tube moved between two levels and then dividing the volume between the two levels by the time taken to move between the two levels to derive the flow rate. This type of testing device does not measure instantaneous flow rates which could fluctuate wildly and could even on an oscilloscope would resemble "grass". The improvement provided by U.S. Pat. No. 4,938,072 was to reject any flow rate if a logic means sensed that the second level had been reached too quickly and let the flow go on to a third level so another calculation of flow rate could be made; this again being timed and rejected depending on how many detectors were used. A flow rate reading is nevertheless a spot reading whose accuracy with pulsed flow rates depends on whether the uppermost level is reached at the start or at the end of a pulse. Of course it would be possible to use a greater volume between the levels but then a reading would take a long period of time. There is another difficulty with testing devices using the time of passage between two levels in a single tube, which is that the tube has to be drained between each spot reading and during this drainage no reading can be taken so that even letting the testing device run continuously gives no more than a series of spot readings, each of which has a tolerance with respect to accuracy.